This invention pertains to the radio communication art and, more particularly, to a system which automatically maintains a uniform delay for simultaneous broadcast of information or message signals by a plurality of transmitter sites.
Simultaneous broadcast, or simulcast, systems are well known in the radio transmission art. In such systems, a plurality of remotely sited transmitters simultaneously broadcast identical audio, or data message signals at a particular carrier frequency. By having one transmitter for each zone, which is a part of a geographical area, maximum signal coverage for the given geographical area is provided. A problem with such systems occurs, however, when a portable radio happens to be positioned between two transmitting sites such that it receives a nearly equal strength carrier signal from each. In this situation, it is important that the message signal from the two transmitters be synchronized in time, otherwise message signal intelligibility may be lost.
Known prior art systems generally have dealt with the problem of different time delays (t.sub.1, t.sub.2, . . . t.sub.N) by utilizing fixed time delays at the central controller to provide a uniform delay to each base station. These fixed delays are preset at the time of equipment installation and alignment, adding a long, fixed time delay to short-delay interconnect links and adding a short, fixed time delay to long-delay interconnect links. Once set, however, the fixed delays remain constant even though the interconnect links' related RF or phone-line equipment may in fact change due to aging or outright substitution. As a result, a technician capable of re-aligning the equipment must be dispatched to diagnose and re-adjust the appropriate fixed delay to bring the disturbed interconnect link back into time synchronization. Such a maintenance process is expensive, time-consuming, and disruptive to the simulcast system.
One known prior art system has compensated for this problem by establishing the following arrangement. An audio signal to be simulcast is sent from a control center to various remotely-sited transmitters. The control center, upon establishing interconnect links between itself and each of the simulcast transmitter sites, merely recalls the appropriate, predetermined time delay value stored in memory to provide the delay needed for signals carried over a given interconnect link. However, such an approach is ineffective for compensating varying amounts of delay caused by more than one possible interconnect link between the control center and a particular remotely-sited transmitter. Quite often, alternate links may be necessary because of inclement weather, interference, or other equipment difficulties. Such problems exist whether the interconnection link is an RF link or a phone-line link. Moreover, because such systems rely on predetermined, stored values of time delay in a memory bank located at the central controller, these systems operate in an open-loop fashion, unable to fully compensate for the amount of audio delay encountered in a new, alternate interconnect link. Such an approach, therefore, does not totally eliminate costly periodic maintenance.
Accordingly, there exists a need for an improved automatic synchronization system for simulcast systems capable of remotely adjusting the total delay to each base site transmitter such that each interconnect link is effectively compensated in closed-loop fashion to automatically provide a uniform, time-synchronized signal.